1. Field of the Invention:
The present invention relates to a doctor blade apparatus in a keyless printing machine, and particularly to a doctor blade apparatus in a keyless inking arrangement capable of changing a total amount of ink supply as appropriate; for example, a doctor blade apparatus in a keyless inking arrangement which uses high-viscosity ink (so-called hard ink) used in an offset printing machine or low-viscosity ink (so-called soft ink).
2. Description of the Related Art:
Generally, inking arrangements of an ink-fountain type are known to use high-viscosity ink and are divided broadly into two categories, an ink-key type and a blade type.
In an inking arrangement of the ink-key type (not shown), an ink fountain is provided with a plurality of ink keys whose tips face the smooth circumferential surface of a fountain roller. The tips of the ink keys are each moved with respect to the circumferential surface of the fountain roller so as to maintain appropriate gaps therebetween. As the fountain roller rotates, ink adhering to the circumferential surface of the fountain roller is drawn out as appropriate through the gaps.
In an inking arrangement of the blade type (not shown), there is used a blade that is substantially as long as a fountain roller. In contrast to a rigid ink key, the blade has an elastic edge, which faces the smooth circumferential surface of the fountain roller. An appropriate gap is previously established between the blade edge and the fountain roller surface. A separately provided member is adapted to move the blade edge toward or away from the fountain roller surface to thereby adjust the gap as appropriate. As the fountain roller rotates, ink adhering to the circumferential surface of the fountain roller is drawn out as appropriate through the gap.
In these inking arrangements of the ink-fountain type, in principle, tips of the ink keys and the edge of the blade are not pressed against the circumferential surface of the fountain roller.
A conventional doctor blade apparatus in a keyless inking arrangement and ink used therewith are disclosed, for example, in "Newspaper Printing Handbook" (The Japan Newspaper Publishers and Editors Association, Apr. 10, 1997, pp. 67, 68, 178, 189). The doctor blade apparatus is adapted to supply ink such that an ink film formed newly on the circumferential surface of a form roller always becomes uniform irrespective of local variations in ink consumption derived from irregular distribution of printed image patterns.
According to the above publication, in the doctor blade apparatus, the edge of the doctor blade is pressed against a metering roller of resin having fine pits and projections formed on its surface, thereby scraping excess ink off the surface and thus forming a substantially uniform film of ink. The thus-formed uniform film of ink is transferred onto the form roller. Also, ink used with the keyless inking arrangement is generally low in viscosity as compared to ink used with inking arrangements of the ink-fountain type and of an ink-pump type employed in ordinary offset printing machines.
The doctor blade apparatus disclosed in the above publication is of the keyless inking arrangement that uses so-called keyless ink having low viscosity, generally a Laray viscosity of about 20 to 30 poise or about 35 to 50 poise. According to an inking sequence, an excessive amount of ink is transferred from an upstream roller onto the circumferential surface of the metering roller. The edge of the doctor blade pressed against the circumferential surface of the metering roller scrapes excess ink off the surface to thereby transfer ink onto the form roller in the form of a substantially uniform film. A surface portion of the metering roller is mainly made of a resin, for example, and has numerous fine pits and projections formed thereon so as to receive ink as appropriate.
Prior art for changing a total amount of ink supply through action of a doctor blade apparatus is disclosed in, for example, Japanese Patent Publication No. 6-75963.
In the doctor blade apparatus disclosed in the above publication, a base of a blade holder for holding a doctor blade is rotatable, and the position of the base is adjustable so as to adjust the distance between the doctor blade and the center of a metering roller. Through adjustment of the distance and the angle of the base, the state in which the edge of the doctor blade is pressed against the circumferential surface of the metering roller is adjusted within a range from a tangential state in which the edge follows rotation of the metering roller to a tangential state in which the edge counters the rotation. This arrangement enables use of inks of different viscosities.
The above doctor blade apparatus disclosed in "Newspaper Printing Handbook" (The Japan Newspaper Publishers and Editors Association, Apr. 10, 1997, pp. 67, 68, 178, 189) has a drawback in that adjusting the degree of scraping ink with the edge of the doctor blade or adjusting the thickness of a formed ink film is impossible. Accordingly, the density of image printed on paper (hereinafter, simply referred to as "printing density") varies depending on condition or wear of the edge of the doctor blade or condition of the circumferential surface of the metering roller, or varies with their time-course change.
The above problem, coupled with the following problems, impairs printing quality.
(1) Because of inevitable adverse factors, such as nonuniformity among manufactured metering rollers in material for a surface portion and in machined state of the surface, transfer of ink differs among metering rollers, resulting in nonuniform printing density.
(2) In the case of color printing, color inks differ in consumption in order to obtain a desired printing density, thus failing to achieve color balance.
(3) As printing speed varies, the amount of transfer of ink varies, causing variation in printing density.
(4) Operation-related variation in temperature causes ink viscosity to vary; thus, the amount of transfer of ink varies, causing variation in printing density.
The above inking arrangement disclosed in Japanese Patent Publication No. 6-75963 is not devised in terms of a doctor blade and a blade holder. Through adjustment of rotational angle of the base that carries the doctor blade and the blade holder and through position adjustment of the base, the angle and the force of pressing the edge of the doctor blade against the circumferential surface of a metering roller are varied. The correlation between the angle and the force of pressing the edge of the doctor blade against the circumferential surface of the metering roller is very difficult to obtain. In other words, in order to increase or decrease the pressing force for varying the degree of scraping ink with the edge, the doctor blade must be deflected to a relatively large extent, causing great variation in the pressing angle.
Similarly, when the pressing angle is increased or decreased, the pressing force varies to a relatively large extent. In other words, at an appropriate pressing angle, a pressing force appropriate for the angle is difficult to obtain. As a result, inking varies with a resultant failure to obtain stable printing density. Also, scraping high-viscosity ink induces a large resistance force, which may cause deformation of the doctor blade. Thus, defective printing is likely to occur.